Project Summary/Abstract This proposal requests funding for a phase IIa clinical trial of rifampin, an FDA-approved antibiotic, for safety and efficacy as a treatment for idiopathic infantile hypercalcemia (IIH). IIH is an uncommon metabolic condition characterized by elevated plasma levels of the activated form of vitamin D, calcitriol, and consequently increased intestinal absorption of calcium that leads to hypercalcemia and hypercalciuria. Although IIH typically presents in infancy, patients manifest a life-long defect in vitamin D metabolism that results in hematuria, renal calcification, and renal insufficiency. Biallelic inactivating mutations of CYP24A1, the gene encoding the 24- hydroxylase enzyme that represents the principal pathway for inactivation of vitamin D metabolites, cause the most common and severe form of IIH. The loss of this degradative pathway allows plasma levels of calcitriol to rise excessively and overcomes negative feedback mechanisms that should downregulate production of calcitriol. There is at present no specific long-term treatment for patients with CYP24A1 mutations and IIH, and conventional care consists of minimizing sunlight exposure, a low calcium diet, and avoidance of vitamin D-rich foods and vitamin D supplements, but this approach does not reduce the risk of renal calcification and renal insufficiency. Thus, there is a significant unmet medical need for safe and effective treatments for this disorder. We have compelling data supporting a novel therapeutic approach that repurposes rifampin to induce expression of CYP3A4, an enzyme that is expressed in the liver and intestine and when over expressed provides an alternative pathway for inactivation of vitamin D metabolites. The long-term goal of this project is to use knowledge of enzymatic pathways that regulate vitamin D metabolism to develop novel strategies for medical treatment of patients with IIH and other forms of hypercalciuria and nephrolithiasis that are associated with elevated plasma levels of calcitriol. The objective in this application is to determine the optimal safe and effective dose of rifampin that normalizes serum and urine levels of calcium and reduces intestinal absorption of calcium (primary outcomes). Our complementary goals are to evaluate the extent to which these primary outcomes are related to plasma levels of rifampin, induction of CYP3A4, polymorphisms in the CYP3A4 gene, and changes in plasma levels of vitamin D metabolites. Our central hypothesis is that induction of CYP3A4 by rifampin will reduce levels of calcitriol, in the plasma and/or intestine, and thereby decrease intestinal absorption of calcium. We expect that overall benefits will be strongly associated with the extent of CYP3A4 induction. Our secondary aim is to use our results to drive a clinical trials simulator that will inform our development of a protocol for a larger, Phase IIb pivotal trial of rifampin for IIH. We have access to the necessary study subjects and the expertise and resources to pursue these studies. Our approach is innovative because it proposes to repurpose a well-characterized and safe medication to a new role as a primary therapy for a disorder that currently lacks an effective treatment.